Spiral taps are used widely as tools for forming internal thread. (a) The spiral tap includes a thread part being provided with an external thread corresponding to an internal thread to be worked, a helical flute(s) dividing the external thread, and a cutting edge(s) formed along the helical flute. (b) By screwing the thread part into a prepared hole formed in a workpiece, the cutting edge cuts i.e., forms the internal thread on an inner circumferential surface of the prepared hole. At the same time, chips are discharged toward a shank via the helical flute.
The example of such spiral tap is described in Patent Gazette 1. A lead of the helical flute gradually increases from a tip end of the thread part to a flute terminal part positioned at side of a shank. A helix angle is small at the tip end part at which the lead is small. Accordingly, a rake angle of the cutting edge is large at the leading part positioned at the tip end to thereby provide an excellent cutting quality and to make the chip to be curled well. Meanwhile, at side of the shank where the lead is large, the chips are discharged rapidly toward side of the shank with guided by the helical flute so that a chip clogging is suppressed satisfactorily.
Patent Gazette 1: Japanese Utility Model Examined Publication No. S54-2476
However, the conventional spiral tap with the helical flute having the gradually increased lead has following problem. That is, because the lead of the helical flute, that is the helix angle varies even at the leading part that cuts the internal thread, the cutting quality and the shape of chips (curled shape, etc.) varies depending on position in the helical flute. Consequently, the cutting performance and the chip discharge performance become unstable, so that the spiral tap may be broken at an early stage due to the chip clogging depending on the working i.e., processing conditions.